chore: align repo scaffolding with the package standard

[TheGreatAxios]

Closes CL-9393

Summary

• LICENSE: the Interchange LGPL-2.1 text, byte-identical across @corbits repos (was the FSF text); add CLA.md and the cla.yml CLA Assistant workflow
• package.json: drop main/types (exports covers Node 24 and Bun); engines adds bun; files lists README.md before LICENSE as in every repo; typecheck is plain tsc --noEmit (now covering e2e/), typecheck:e2e is gone, test adds --pass-with-no-tests, and a new check runs typecheck, lint, format check and unit tests. Key order is oxfmt's
• tsconfig.json and tsconfig.build.json follow the standard templates; delete tsconfig.e2e.json and tsconfig.typecheck.json. The typecheck config gains exactOptionalPropertyTypes, isolatedModules, moduleDetection and resolveJsonModule, loses noFallthroughCasesInSwitch, and uses Bun types only
• exactOptionalPropertyTypes fixes (no behavior change):
  • src/client/client.ts: optional fields on GmailRequest and the GmailClient method options accept undefined, since the tools forward optional inputs as-is
  • src/tools/models.ts: messageMetadata reads the subject and from headers once and omits the key when absent
  • e2e/helpers.ts: build the signed Request from the source request or the URL string in separate branches
• .gitignore: the standard list plus .local/, which holds the live suite's Gmail refresh token
• ci.yml: runs on pull requests and pushes to main (was every branch push); bun run check, then a pack-and-import smoke under Node 24 that installs the tarball and checks createGmailTools, createGmailClient and GmailApiError (the old smoke imported ./dist directly); drops the single-entry matrix
• CONTRIBUTING.md: the standard template plus a CLA line and a shortened Live Gmail checks section (same env vars and fixture rules)
• AGENTS.md: rewritten from the placeholder to purpose, layout, rules and local development
• Delete CLAUDE.md (Bun's generated boilerplate)
• README: rename the "Tools" section to "Reference"

Stacked on #10.

• CONTRIBUTING gets a Releasing section: releases are manual, as in Interchange (npm version, push tags, gh release create --generate-notes, npm publish).

Commits

1. refactor: type optional Gmail fields for exactOptionalPropertyTypes
2. chore: use the Interchange LICENSE text and add the CLA
3. build: align package.json and tsconfig with the package standard
4. ci: run check and a Node pack smoke
5. docs: trim CONTRIBUTING, rewrite AGENTS.md, drop CLAUDE.md

Checks

bun run build, bun run check and bun run test:e2e (live suites skipped) pass; the Node pack smoke passes locally.